Method and apparatus for fabricating circuit-forming-substrate and circuit-forming-substrate material

ABSTRACT

A method for manufacturing a printed wiring board, comprising the step of forming a hole by an energy beam such as a laser beam, wherein formation of a resin film by a substrate-material resin oozing to the inner-wall surface of a hole is prevented, by lowering the water-absorption percentage of a substrate material through the dehumidifying step as the preprocess of the hole-forming step for forming a through-hole or non-through-hole for interconnecting circuits formed on both sides or in multiple layers, thereby it is possible to realize high-quality hole-formation by preventing a defective resin film formation and obtain a high-reliability printed wiring board.

TECHNICAL FIELD

[0001] The present invention relates to a method for manufacturing aquality printed wiring board of high reliability using an energy beam,such as a laser beam, for hole-formation. Manufacturing equipment andboard material for the printed wiring board are also included in theinvention.

BACKGROUND ART

[0002] In order to meet the needs in the recent electronic applianceindustry for mounting components in high density in a compact space, theincreasing number of printed wiring boards is shifting from theconventional single-sided version towards the double-sided and thosehaving multi-layers. Thus the development efforts are focused onproducing a high density printed wiring board that can accept as manywiring circuits thereon.

[0003] For forming a fine hole (approximately 200 μm diameter, forexample) at a high speed, use of an energy beam, such as a laser beam,is being studied in place of the conventional through-hole making with adrill (Kiyoshi Takagi: “Significant Trends, the Development of BuildupMultilayered PWBs”, Surface Mounting Technology, No. 1, pages 2-10(1997); and other publications). Examples of the board material includea film of thermosetting resin at B-stage state containing uncuredcomponent, and a composite material of woven, or nonwoven, organic orinorganic fabric and thermosetting resin.

[0004] A conventional process comprises a hole forming step and aconnection means forming step, as shown in FIG. 5. The aim of forming ahole in a wiring board is to interconnect the circuits provided on thesurfaces or in the inner layers of a wiring board. The interconnectionmeans is formed by filling the hole with a conductive paste or byapplying a plating process. For example, a both-surface type printedwiring board is produced by first filling a through-hole of the boardwith a conductive paste containing electro-conductive particles by wayof a printing or the like process, and then copper foil is provided onboth surfaces of the board to be unitized by thermal compression. Thecopper foils are then made to have specific patterns. The board materialis required to become soft or molten at the thermal compression; so, aB-stage thermosetting resin containing uncured component or athermoplastic resin is used.

[0005] When forming a hole in a wiring board of the above describedmaterial with an energy beam, such as a laser beam, a resin 2 and awoven, or nonwoven, fabric 3 contained in the board material 1 areheated by the beam and sublimated to be scattering around, and a hole 5is formed. Resin material existing around the hole is also softened andmolten by a heat of low-energy beam that does not contribute toperforating a hole, which resin material oozing out from the wallsurface of the hole forms a thin film to cover a part or the whole ofthe wall surface, as illustrated in FIG. 6(a).

[0006] In a case when the resin material of the board absorbs moisture,it is soften and molten more easily by the heat of an energy beam, andvolume of the oozing resin increases. As volume of the absorbed moistureincreases, the oozing resin comes into contact to each other as shown inFIG. 6(b), which is then solidified and contracted after cooling to forma thin resin film 10, eventually clogging the through-hole 5. Thisphenomenon becomes significant with the holes of small diameter.

[0007]FIG. 7 is a chart showing a concept of relationship betweenpercentage of water absorbed in the board material 1 and rate of rejectsdue to formation of resin film 10. As soon as the percentage of waterabsorbed in the board material (water absorbed versus board material 1in terms of weight; hereinafter referred to as wt %) exceeds a certainpoint the reject rate sharply picks up. The reject rate and thepercentage of absorbed moisture in the threshold value vary depending onthe hole diameter and the board material 1. In the through-holes 5having such a resin film 10 formed therein, troubles in the electricalinterconnection often arise in a subsequent process step. Examples ofthe trouble include; no electrical connection accomplished between thecircuits of both surfaces or layers of a wiring board because ofincomplete formation of the conductive material or the plating throughthe opposite surface of a board, the hole not filled with sufficientamount of conductive material, a high contact resistance in anaccomplished interconnection, etc.

[0008] The present invention aims to offer a manufacturing method, amanufacturing equipment and a board material for producing reliableprinted wiring boards having quality holes. In accordance with theinvention, rejects due to the formation of resin film is eliminated, orat least reduced, to an improved production yield rate.

DISCLOSURE OF THE INVENTION

[0009] The present invention relates to a method for manufacturing aprinted wiring board. The method comprises steps of forming athrough-hole, or a non-through-hole, with an energy beam in a boardmaterial of plate form, or sheet form, composed mainly of either athermoplastic resin or a thermosetting resin containing uncuredcomponent, or a mixture of both, or in a board material of plate form,or sheet form, made of a woven, or nonwoven, fabric impregnated with amaterial composed mainly of either the thermoplastic resin orthermosetting resin containing uncured component, or a mixture of both,forming connection means in the through-hole, or non-through-hole,provided by the hole-forming step, for electrically interconnecting onesurface and the reverse surface of the board material, and dehumidifyingthe board material before the hole-forming step.

[0010] Under the manufacturing process in accordance with the presentinvention, production yield rate is improved through a reduced rate ofrejects due to the formation of resin film during the hole-forming step.A high-quality hole-formation is realized and a high-reliability printedwiring board is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a flowchart showing the process in accordance with afirst exemplary embodiment of the present invention.

[0012]FIG. 2(a) - (f) are cross sectional views illustrating the processof manufacturing a printed wiring board in accordance with the firstembodiment of the present invention.

[0013]FIG. 3(a) - (c) show process flowcharts in accordance with asecond exemplary embodiment of the present invention.

[0014]FIG. 4(a), (b) show laminating processes in accordance with athird exemplary embodiment of the present invention.

[0015]FIG. 5 is a flowchart showing a conventional process.

[0016]FIG. 6(a), (b) show cross sectional views of a hole formed by aconventional procedure.

[0017]FIG. 7 is a characteristics chart showing the- relationshipbetween the rate of rejects due to formation of resin film and thepercentage of water absorbed in board material.

BEST MODE FOR CARRYING OUT THE INVENTION

[0018] A method for manufacturing a printed wiring board in accordancewith the present invention comprises the steps of forming athrough-hole, or a non-through-hole, with an energy beam in a boardmaterial of plate form, or sheet form, composed mainly of either athermoplastic resin or a thermosetting resin containing uncuredcomponent, or a mixture of both, or in a board material of plate form,or sheet form, made of a woven, or nonwoven, fabric impregnated with amaterial composed mainly of the thermoplastic resin or thermosettingresin containing uncured component, or a mixture of both, formingconnection means in the through-hole, or non-through-hole, provided bythe hole-forming step for electrically interconnecting one surface andthe reverse surface of the board material, and dehumidifying the boardmaterial before the hole-forming step. With the above method ofmanufacture, rate of rejection due to the formation of resin film at thehole-forming step is reduced, and high-quality hole-formation isrealized to obtain a high-reliability printed wiring board.

[0019] Preferably, the dehumidifying step is a hot air drying process.Thereby, a board material of high moisture absorption rate can be surelydehumidified with ease.

[0020] Preferably, temperature of the hot air drying process is 50° C.or higher, and not higher than a limitation of temperature and timeperiod in which the gel time of thermosetting resin does not startshifting. Thereby, a board material of high moisture absorption rate canbe surely dehumidified with ease in a relatively short period of time,without inviting a change in the material property of the boardmaterial.

[0021] Preferably, the dehumidifying step is a vacuum drying process.Thereby, a board material of high moisture absorption rate can be surelydehumidified with ease.

[0022] Preferably, the vacuum drying is performed without accompanying aheating. Thereby, a board material of high moisture absorption rate canbe surely dehumidified easily, without inviting a change in the materialproperty of the board material.

[0023] Preferably, the vacuum drying process is accompanied by aheating. Thereby, a board material of high moisture absorption rate canbe dehumidified efficiently within a short period of time.

[0024] Preferably, the heating temperature is at least lower thanboiling point of a solvent contained in a resin of board material, andlower than a limitation of temperature and time period in which the geltime of thermosetting resin does not start shifting. Thereby, a boardmaterial of high of moisture absorption rate can be dehumidified withoutinviting bubbling of impregnated resin which might be caused byevaporation of solvent, and without inviting a change in the materialproperty of the board material.

[0025] Preferably, degree of the vacuum ambient is not lower than 100Torr. Thereby, the dehumidifying is conducted efficiently within a shortperiod of time.

[0026] Preferably, the dehumidifying is performed at once with two ormore pieces of board materials stacked together direct to each other.Thereby, the plurality of board materials may be efficientlydehumidified, which is a substantial advantage in volume production.

[0027] Preferably, the dehumidifying step brings the content of water inthe board material to be not larger than 1% in terms of weightpercentage. Thereby, rejects due to the formation of resin film isprevented, and high-quality hole-formation is realized. The reliabilitylevel of a printed wiring board is improved.

[0028] A method for manufacturing a printed wiring board in accordancewith the present invention comprises the steps of forming athrough-hole, or a non-through-hole, with an energy beam in a boardmaterial of plate form, or sheet form, composed mainly of either athermoplastic resin or a thermosetting resin containing uncuredcomponent, or a mixture of both, or in a board material of plate form,or sheet form, made of a woven, or nonwoven fabric, impregnated with amaterial composed mainly of either said thermoplastic resin orthermosetting resin containing uncured component, or a mixture of both,forming connection means in said through-hole, or non-through-hole,provided by said hole-forming step for electrically interconnecting onesurface and the reverse surface of the board material, and suppressingmoisture absorption of said board material before said hole-formingstep. With the above method of manufacture, moisture absorption duringstorage on shelf can be suppressed; as a result, rejects due to theformation of resin film in the hole-forming step is reduced, andhigh-quality hole-formation is realized. The reliability of a printedwiring board is improved.

[0029] Preferably, the step of suppressing moisture absorption isprovided in either between the dehumidifying step and the hole-formingstep or in the hole-forming step, or in both steps. By so doing,moisture absorption in the hole-forming step is also suppressed.

[0030] Preferably, the process of suppressing the moisture absorption isconducted by storing a board material on shelf in a low humidityatmosphere. By so doing, the absolute humidity goes down and thepercentage of saturatal water content in a board material is lowered.

[0031] Preferably, ambient temperature of the hole-forming step ishigher than dew point temperature of the low humidity atmosphere. Undersuch environment, there will be no dew condensation on a board material.

[0032] Preferably, the low humidity atmosphere is a vacuum ambient.Under such state, the absolute humidity can easily be lowered and thepercentage of saturatal water content of a board material can belowered.

[0033] Preferably, the vacuum ambient is a hermetically sealed spacewhich is evacuated after a board material is lodged in. By so doing, theabsolute humidity can easily be lowered and maintained.

[0034] Preferably, the low humidity atmosphere is a hermetically sealed,or a circulating, space substituted with dry nitrogen. This provides asimple method for suppressing the moisture absorption during storage onshelf.

[0035] Preferably, the process of suppressing moisture absorption keepscontent of water in a board material to be 1% or lower in terms ofweight percentage. By so doing, rejects due to the formation of resinfilm is prevented, and high-quality hole-formation is realized. Thereliability level of a printed wiring board is improved.

[0036] Preferably, the low humidity atmosphere is an atmosphere in whichthe partial pressure of water vapor is not larger than 10 mmHg. Undersuch atmosphere, content of water in the board material after storage onshelf never goes higher than 1% in terms of weight percentage,regardless of the time period it stayed on shelf.

[0037] A method for manufacturing a printed wiring board in accordancewith the present invention comprises the steps of forming athrough-hole, or a non-through-hole, with an energy beam in a boardmaterial of plate form, or sheet form, composed mainly of athermoplastic resin or a thermosetting resin containing uncuredcomponent, or a mixture of both, -or in a board material of plate form,or sheet form, made of a woven, or nonwoven, fabric impregnated with amaterial composed mainly of said thermoplastic resin or thermosettingresin containing uncured component, or a mixture of both, and formingconnection means in said through-hole, or non-through-hole, provided bysaid hole-forming step for electrically interconnecting one surface andthe reverse surface of a board material, wherein said board materialstored on shelf is carried to the hole-forming step within a time periodin which the water content in said board material does not go beyond 1%in terms of weight percentage. Under the above described manufacturingmethod, the moisture absorption may be suppressed only through thesimple control of time.

[0038] Preferably, the board material is laminated at least on bothsurfaces with sheet film having a high anti-moisture permeability untilit is carried to the hole-forming step. By so doing, the moistureabsorption can be suppressed regardless of the absolute humidity ofstorage environment.

[0039] Preferably, the board material is laminated at both surfaces withfilm sheets having a high anti-moisture permeability whose size isgreater than the shape of board material, and said film sheets coveringthe both surfaces are sealed together by heat in the area exceedingouter edge of the board material so that the board material is isolatedfrom the outside air until it is carried to the hole-forming step. By sodoing, moisture absorption of a board material from the end can also besuppressed during storage on shelf.

[0040] Preferably, the sheet film having a high anti-moisturepermeability is of polyethylene terephthalate. It is inexpensive andrecyclable.

[0041] Preferably, the film is provided with a metal film formed on thesurface by deposition or the like method. The use of such a filmsuppresses the moisture absorption during storage on shelf moreeffectively.

[0042] Preferably, the metal film is of aluminum. It is inexpensive.

[0043] Preferably, the energy beam is a laser beam. The laser beam has agood concentration property on a board material, and it can be scannedeasily with an optical device or the like means.

[0044] Preferably, the laser beam is a CO₂ laser beam. It provides ahigh energy at low cost.

[0045] Preferably, the process of filling the through-hole, ornon-through-hole, provided by the hole-forming step with a pastecontaining conductive particles is included in the connection meansforming step. By so doing, rejects due to the formation of resin film inthe hole-forming step can be reduced, and high-quality hole-formation isrealized to an improved reliability of a printed wiring board.

[0046] Preferably, the process of providing a board material filled witha paste containing conductive particles with a metal foil on onesurface, or both surfaces, to be unitized together by thermalcompression is included in the connection means forming step. By sodoing, rejects due to the formation of resin film in the hole-formingstep can be reduced, and high-quality hole-formation is realized to animproved reliability of a printed wiring board.

[0047] Preferably, the process of providing the through-hole, ornon-through-hole, formed by the hole-forming step with a plating ofconductive metal is included in the connection means forming step. By sodoing, rejects due to the formation of resin film in the hole-formingstep can be reduced, and high-quality hole-formation is realized to animproved reliability of a printed wiring board.

[0048] Preferably, the thermosetting resin is an epoxy resin. It has animproved anti-moisture property.

[0049] Preferably, the woven, or nonwoven, fabric is made mainly oforganic fiber material. The use of organic fiber, which has a relativelyclose material property to the resin, makes the hole-formation with anenergy beam easy.

[0050] Preferably, the organic fiber material is mainly of an aromaticpolyimide fiber. It makes the hole-formation with an energy beam easy,and the printed wiring board lighter in weight and more reliable.

[0051] An equipment for manufacturing a printed wiring board inaccordance with the present invention forms a through-hole, ornon-through-hole, in a board material of printed wiring board with anenergy beam generated from an energy beam generating unit, wherein thedehumidifying means is provided in a section for feeding said boardmaterial. With the above described manufacturing equipment, rejects dueto the formation of resin film in the hole-forming step is reduced, andhigh-quality hole-formation is realized to an improved reliability of aprinted wiring board.

[0052] Preferably, the dehumidifying means is a hot air dryer. Thereby,a board material of high moisture absorption can be surely dehumidifiedwith ease.

[0053] Preferably, temperature of the hot air drying is not lower than50° C., and not higher than a limitation of temperature and time periodin which the gel time of board material does not start shifting.Thereby, a board material of high moisture absorption rate can be surelydehumidified with ease in a relatively short period of time, withoutinviting a change in the material property of the board material.

[0054] Preferably, the dehumidifying means is a vacuum drying unit.Thereby, a board material of high moisture absorption rate can be surelydehumidified with ease.

[0055] Preferably, the vacuum drying unit does not accompany a heating.Thereby, a board material of high moisture absorption rate can be surelydehumidified with ease, without inviting a change in the materialproperty of the board material.

[0056] Preferably, the vacuum drying unit accompanies a heating.Thereby, a board material of high moisture absorption rate can bedehumidified efficiently within a short period of time.

[0057] Preferably, heating temperature of the vacuum drying unit is setto be at least lower than boiling point of a solvent contained in resinof the board material. Thereby, a board material of high moistureabsorption rate can be dehumidified without inviting a bubbling of theimpregnated resin, which bubbling is caused by an evaporating solvent,and a change in the material property of the board material.

[0058] Preferably, vacuum degree in the vacuum drying unit is set to benot lower than 100 Torr. Thereby, a board material is surelydehumidified efficiently within a short period of time.

[0059] Preferably, the dehumidifying means is a unit that is capable ofdehumidifying two or more pieces of board materials at once stackingtogether direct to each other. Thereby, the plurality of board materialsmay be efficiently dehumidified, which is a substantial advantage in thevolume production.

[0060] Preferably, the dehumidifying means is a unit that is capable ofreducing the content of water absorbed in the board material to be lowerthan 1% in terms of weight percentage. Thereby, rejects due to theformation of resin film is prevented, and high-quality hole-formation isrealized to an improved reliability of a printed wiring board.

[0061] An equipment for manufacturing printed wiring board in accordancewith the present invention forms a through-hole, or non-through-hole, ina board material for forming printed wiring board with an energy beamgenerated from an energy beam generating unit, wherein means forsuppressing moisture absorption is provided either in a section forfeeding said board material or in a section for forming the hole, or inboth sections. With the above described manufacturing equipment, rejectsdue to the formation of resin film in the hole-forming step is lowered,and high-quality hole-formation is realized to an improved reliabilityof a printed wiring board.

[0062] Preferably, in a manufacturing equipment for forming athrough-hole, or non-through-hole, in a board material for formingprinted wiring board with an energy beam guided from an energy beamgenerating unit, means for suppressing the moisture absorption isprovided either in a section for feeding said board material or in asection for forming the hole, or in both sections. By so doing, themoisture absorption during the hole-forming step can also be suppressed.

[0063] Preferably, the means for suppressing moisture absorption is aunit that stores a board material on shelf in a low humidityenvironment.

[0064] Preferably, the low humidity environment is a vacuum state. Inthe vacuum state, the absolute humidity goes low and the percentage ofsaturatal water content of a board material can be reduced.

[0065] Preferably, the vacuum state is a sealed space which is evacuatedafter a board material is lodged in. In this state, the absolutehumidity can easily be lowered and maintained there.

[0066] Preferably, the low humidity environment is a hermetically sealedspace, or a circulating environment, substituted with dry nitrogen. Thisprovides a simple method of suppressing the absorption of moistureduring storage on shelf.

[0067] Preferably, the means for suppressing moisture absorption is aunit that keeps the content of water in a board material to be lowerthan 1% in terms of weight percentage. With such means, rejection due tothe formation of resin film is prevented, and high-qualityhole-formation is realized to an improved reliability of a printedwiring board.

[0068] Preferably, the low humidity environment is an environment wherethe partial pressure of water vapor is not larger than 10 mmHg. Undersuch environment, the content of water in a board material never goeshigher than 1% in terms of weight percentage, regardless of the timeperiod it stays on shelf.

[0069] A material for manufacturing printed wiring board in accordancewith the present invention is a board material of plate form, or sheetform, composed mainly of a thermoplastic resin or a thermosetting resincontaining uncured component, or a mixture of both, or a board materialof plate form, or sheet form, made of a woven, or nonwoven, fabricimpregnated with a material composed mainly of said thermoplastic resinor thermosetting resin containing uncured component, or a mixture ofboth, wherein content of water in said board material being lower 1% interms of weight percentage. With the above described material, rejectsdue to the formation of resin film in the hole-forming step areavoidable, and high-quality hole-formation is realized to an improvedreliability of a printed wiring board.

[0070] Preferably, the board material for manufacturing printed wiringboard is the one that fits to the hole-forming with an energy beam. Withsuch a material, rejects due to the formation of resin film in thehole-forming step are avoidable, and high-quality hole-formation isrealized to an improved reliability of a printed wiring board.

[0071] Preferably, the thermosetting resin is an epoxy group resin. Ithas an improved anti-moisture property.

[0072] Preferably, the woven, or nonwoven, fabric is made mainly oforganic fiber material. The use of organic fiber, whose materialproperty is relatively close to the resin, makes the hole-forming withan energy beam easy.

[0073] Preferably, the organic fiber material is mainly of an aromaticpolyimide fiber. It makes the hole-forming with an energy beam easy, anda printed wiring board lighter in weight and more reliable.

[0074] Now in the following, exemplary embodiments in accordance withthe present invention are described referring to FIG. 1 through FIG. 4.

Embodiment 1

[0075]FIG. 1 and FIG. 2, respectively, are process flowchart and crosssectional views showing a method for manufacturing printed wiring boardin accordance with a first exemplary embodiment of the presentinvention. The manufacturing process is composed of a dehumidifyingstep, a hole-forming step and a connection means formation step, in theorder. A board material 1 is a composite material of, as shown in FIG.2(a), a thermosetting resin 2 (an epoxy resin, for example) and anonwoven fabric 3 of aromatic polyamide fiber (hereinafter referred toas aramid fiber). The thermosetting resin 2 is not completely cured, butit is in the so-called B-stage state containing non-cured component. Theboard material 1 is a one which is normally called “prepreg”. The boardmaterial 1 of approximately 150 μm thick is dehumidified, and athrough-hole 5 is formed in a laser processing unit with a CO₂ gas laserbeam 4 at a diameter of approximately 200 μm.

[0076] The through-hole 5 is filled with a conductive paste 7 composedmainly of an epoxy resin and conductive particles by a printing method,as shown in FIG. 2(c). Then, as shown in FIG. 2(d), a metal foil 8 isprovided on both surfaces of the board material 1 to be unitizedtogether by thermal compression in the direction of thickness. The metalfoils 8 overlaid on the both surfaces are electrically interconnected bythe conductive paste 7. Each of the metal foils 8 is etched to a certainspecific pattern to complete a printed wiring board, FIG. 2(f).

[0077] Some of the points of significance with the present embodimentare described below. The dehumidifying is conducted either by anindependent dehumidifying unit or dehumidifying means provided in asection of the laser processing unit for feeding board material 1. Inpractice, a board material 1 that has a high moisture absorbing propertymay be dehumidified with hot air or vacuum prior to the hole-formation.

[0078] In the dehumidifying with hot air, it has been confirmed thathigher the hot air temperature the higher the dehumidifying effect, andthe dehumidifying process can be finished within a short period of timewhen the temperature is 50° C. or more. However, in order to preventadverse influence to the following thermal compression process, it isessential that the board material 1 is treated within a limitation oftemperature and time period in which the gel time of thermosetting resin2 contained in the board material 1 does not make a substantial shift.The term, “gel time”, is a parameter representing the progress ofcuring; it is defined by the length of time: A thermosetting resin 2 ofa certain specific weight is placed on a hot plate heated to a certainspecific temperature (170° C., for example), stirred with a Teflon orthe like stick, a period of time until the thermosetting resin 2 ceasesto rope. Normally the dehumidifying with hot air is conducted with theatmospheric air, but the use of dry nitrogen, for example, may lead to amore effective result.

[0079] The dehumidifying in vacuum has an advantage that it candehumidify a board material in the room temperature, without applyingheat. Thus, a board material 1 may be dehumidified without inviting anyshift in the material property, such as the gel time. If a heat is addedduring the vacuum dehumidifying, the effect is enhanced and the timeneeded for dehumidifying will become shorter as compared with the timerequired in the drying with hot air. Attention has to be paid in ordernot to leave adverse influence on the thermal compression process; thatit has to be processed within a limitation of temperature and time inwhich the gel time of thermosetting resin 2 contained in the boardmaterial 1 does not make a substantial shift.

[0080] When the thermosetting resin 2 contains a solvent, the upperlimit of the heating temperature should be lower than boiling point ofthe solvent. If the temperature is not lower than the boiling point,solvent evaporates to result in a bubbling of the thermosetting resin 2,which leaves ill affect to a subsequent process step. Another advantagewith the dehumidifying with vacuum is that it can dehumidify at once twoor more pieces of board materials 1 of a sheet form stacked togetherdirect to each other. Thus, the vacuum dehumidifying takes only a shortperiod of time for dehumidifying, and provides a high operationalefficiency. The process is suitable to the volume production. Theprocess of dehumidifying the board materials 1 in a stacked state is notsuitable to the hot air drying, because the dehumidification effect issmall in that way and takes too much time, allowing the gel time ofboard material 1 to shift significantly. It has been confirmed that thevacuum dehumidifying brings about a satisfactory dehumidification effectwithin a short period of time if grade of the vacuum is not lower than100 Torr. If the vacuum grade is inferior to the value, it takes toomuch time for dehumidification, or hardly any dehumidification isobtainable.

[0081] The lower the moisture contents in a board material 1, whichrepresents the result of the dehumidifying process, the lower the rateof rejects due to the formation of resin film 10. According toexperimental results obtained by the inventor, the rejects areeliminated when the moisture content is not higher than 1 wt % (measuredwith a Karl Fisher moisture meter). However, as the quantity ofthermosetting resin 2 contained in the board material 1 increases, therejects rate due to formation resin film 10 tends to go higher. So, themoisture content in a board material should ideally be almost zero. Inpractice, the moisture content should preferably be 0.6 wt % or less,taking the dispersion in the material property of board material 1 intoconsideration. More preferably, it should be 0.2 wt % or less whentaking into account the stability in a volume production line.

Embodiment 2

[0082]FIG. 3 shows process flowcharts of a printed wiring board inaccordance with a second exemplary embodiment of the present invention.The process consists of means for suppressing moisture absorption, ahole-forming step and a connection means forming step. There arevariations to the process; a case of FIG. 3(a), where the means forsuppressing moisture absorption is placed before the hole-forming step,a case of FIG. 3(b), where the means is placed in between thedehumidifying step of embodiment 1 and the hole-forming step, a case ofFIG. 3(c), where the means is in the hole-forming step, and a case wherethe cases of FIG. 3(b) and FIG. 3(c) are combined. Process steps afterthe hole-forming step are basically the same as those of embodiment 1,therefore detailed description of which steps is omitted.

[0083] The points of significance with the present embodiment aredescribed below. The means for suppressing moisture absorption isconstituted by an independent moisture suppressing unit, or means forsuppressing moisture absorption disposed in a section for feeding theboard material 1 or in the hole-forming step of a laser process unit.Practically, the present embodiment is suitable for such board materials1 whose initial absorption of moisture is relatively low. The moistureabsorption can be suppressed by storing the board materials on shelf ina low humidity atmosphere. An air conditioner is generally used toprovide the low humidity atmosphere. A simpler alternative may beproviding a hermetically sealed space, or a circulating environment,substituted with dry nitrogen. Or, the board material 1 may be placed ina container, and then the container is evacuated after it ishermetically sealed.

[0084] In a case where there is a certain time period for a boardmaterial 1 dehumidified by the process of embodiment 1 until it iscarried to the hole-forming step, it may be stored on shelf in the lowhumidity atmosphere for suppressing the moisture absorption. It isfurther effective if the hole-forming step is conducted in a lowhumidity atmosphere. The point of importance here is that, ifenvironment of the hole-forming step is not low humidity, temperature ofthe low humidity atmosphere needs to be kept higher than dew point ofthe environment of the hole-forming step, in order to avoid dewcondensation on the board material 1.

[0085] Although moisture absorption with a board material 1 is low inthe beginning, or it had been dehumidified to a sufficiently low level,it starts absorbing moisture if stored in a certain environment,eventually reaching a certain percentage of saturatal water content,which is proportionate to absolute humidity of the atmosphere in whichthe board material is stored. Experiments conducted by the inventorreveal that the percentage of saturatal water content can be maintainedbelow 1 wt % so long as it is stored on shelf in an environment wherethe partial pressure of water vapor is not higher than 10 mmHg. Themoisture absorption never go beyond 1 wt % so long as it is stored inthe above described environment, regardless of time span of the storage.

[0086] Another practical way of forming a hole in a board material 1 ina state where the moisture contents are not higher than 1 wt % is toprocessing it before it absorbs moisture to a level higher than 1 wt %during storage on shelf. In this way, means for suppressing the moistureabsorption is not required; it is a simple manufacturing method whichrequires only the control of time.

Embodiment 3

[0087] Other method for suppressing the moisture absorption of boardmaterial 1 is to laminate the both surfaces with a sheet film having ahigh anti-moisture permeability. Laminating the board material 1 atleast on both surfaces with sheet film 11 of anti-moisture permeability,as illustrated in FIG. 4(a), suppresses the moisture absorption. Theanti-moisture effect is enhanced by using sheet films 11 that have agreater size than the board material 1, and sealing them together byheat in an area outer than the board material 1. By so doing, the boardmaterial 1 is isolated from the outside air and the moisture absorptionfrom the end is also suppressed. Polyethylene terephthalate (PET), whichis inexpensive and recyclable, may be a good candidate for the film 11of anti-moisture permeability. The effectiveness of moisture suppressionis further enhanced if the sheet film is provided with a film ofaluminum, or the like metal, deposited on the surface.

[0088] Although in the above described three exemplary embodiments theallowable contents of moisture in the board material have been specifiedto be not higher than 1 wt %, effectiveness of the suppressed moistureabsorption remains as it is in reducing the rejects due to formation ofresin film 10 even if the moisture absorbed is somewhat more than theabove specified value. Although CO₂ laser has been described as anexample of the energy beam, other gas lasers, YAG laser and othersolid-state lasers, eximer lasers, or energy beams other than laser mayof course be used instead for the purpose. Although the descriptionshave been made referring to a double-faced printed wiring board, thepresent technology is of course applicable to the manufacture ofmulti-layered printed wiring boards by repeating the process steps.Instead of the nonwoven fabric, a woven fabric may also be used; thenonwoven fabric or woven fabric may also be formed with an organic fiberother than aramid fiber or with an inorganic fiber material such asglass; a thermoplastic resin may also be used in place of thethermosetting resin; the connection means may be provided also by aninterconnection formed with plating or pressure welding. Thus, theabove-described embodiments are not to be interpreted as limiting.

INDUSTRIAL APPLICABILITY

[0089] Providing a dehumidifying step before the hole-forming stepbrings about-a substantial advantage in improving the manufacturingyield rate through the reduced rejects due to formation of resin film. Ahigh-quality hole-formation is realized in a printed wiring boardmanufactured in accordance with the present invention, and the printedwiring boards thus manufactured are furnished with a high-reliability.

[0090] In addition to the above-described advantages, following effectsare expected.

[0091] As the diameter of a hole goes smaller the oozing resin readilysticks together, which leads to the rejects due to formation of resinfilm. The present invention advances the smallest limit of the holediameter a step further. The high-quality hole-formation is realizedwith the holes of smaller diameters. This technology helps presenthigh-density printed wiring boards at a high manufacturing yield rate.

[0092] Furthermore, as the quantity of resin oozing out from the wallsurface of a hole decreases the quantity of conductive paste to befilled in the hole increases accordingly, and the compression rate ofconductive particles increases in the thermal compression procedure.Thus, high reliability printed wiring boards having low interconnectionresistance are presented.

1. A method for manufacturing a printed wiring board comprising thesteps of: forming a through-hole, or a non-through-hole, using an energybeam in a board material of plate form, or sheet form, composed mainlyof either a thermoplastic resin or a thermosetting resin containinguncured component, or a mixture of both, or in a board material of plateform, or sheet form, made of woven, or nonwoven, fabric impregnated witha material composed mainly of either said thermoplastic resin orthermosetting resin containing uncured component, or a mixture of both;forming connection means in said through-hole, or non-through-hole,provided as a result of said hole-forming step, for electricallyinterconnecting one surface and the reverse surface of a board material;and dehumidifying said board material before said hole-forming step. 2.The method for manufacturing a printed wiring board recited in claim 1,wherein the moisture-proof is conducted by a hot air drying.
 3. Themethod for manufacturing a printed wiring board recited in claim 2,wherein a temperature of the hot air drying is not lower than 50° C.,and within a limitation of temperature and time period in which the geltime of thermosetting resin does not shift.
 4. The method formanufacturing a printed wiring board recited in claim 1, wherein thedehumidifying is conducted by a vacuum drying.
 5. The method formanufacturing a printed wiring board recited in claim 4, wherein thevacuum drying does not accompany heating.
 6. The method formanufacturing a printed wiring board recited in claim 4, wherein thevacuum drying accompanies heating.
 7. The method for manufacturing aprinted wiring board recited in claim 6, wherein a temperature of theheating is lower than boiling point of a solvent contained in a resin ofboard material, and within a limitation of temperature and time periodin which the gel time of thermosetting resin does not shift.
 8. Themethod for manufacturing a printed wiring board recited in claim 4,wherein level of the vacuum is not lower than 100 Torr.
 9. The methodfor manufacturing a printed wiring board recited in claim 4, wherein thedehumidifying is conducted at once for two or more pieces of boardmaterials stacked up directly together.
 10. The method for manufacturinga printed wiring board recited in claim 1, wherein the dehumidifyingstep lowers the content of water in the board material to be not morethan 1% in terms of weight percentage.
 11. A method for manufacturing aprinted wiring board comprising the steps of: forming a through-hole, ora non-through-hole, using an energy beam in a board material of plateform, or sheet form, composed mainly of either a thermoplastic resin ora thermosetting resin containing uncured component, or a mixture ofboth, or in a board material of plate form, or sheet form, made ofwoven, or nonwoven, fabric impregnated with a material composed mainlyof either said thermoplastic resin or thermosetting resin containinguncured component, or a mixture of both; forming connection means insaid through-hole, or non-through-hole, provided as a result of saidhole-forming step, for electrically interconnecting one surface and thereverse surface of a board material; and suppressing moisture absorptioninto said board material before said hole-forming step.
 12. The methodfor manufacturing a printed wiring board recited in claim 11, whereinthe process of suppressing moisture absorption is provided either inbetween the dehumidifying step and the hole-forming step or in thehole-forming step, or in both of the steps.
 13. The method formanufacturing a printed wiring board recited in claim 11, wherein theprocess of suppressing moisture absorption is storing a board materialon shelf in a low humidity atmosphere.
 14. The method for manufacturinga printed wiring board recited in claim 13, wherein an environmentaltemperature of the hole-forming step is higher than dew point of the lowhumidity atmosphere.
 15. The method for manufacturing a printed wiringboard recited in claim 14, wherein the low humidity atmosphere is avacuum state.
 16. The method for manufacturing a printed wiring boardrecited in claim 15, wherein the vacuum state is a hermetically sealedspace which is evacuated after a board material is lodged in.
 17. Themethod for manufacturing a printed wiring board recited in claim 13,wherein the low humidity atmosphere is a sealed, or circulating,atmosphere substituted with dry nitrogen.
 18. The method formanufacturing a printed wiring board recited in claim 11, wherein theprocess of suppressing moisture absorption keeps content of water in aboard material to be not more than 1% in terms of weight percentage. 19.The method for manufacturing a printed wiring board recited in claim 18,wherein the low humidity atmosphere is an atmosphere where the partialpressure of water vapor is not larger than 10 mmHg.
 20. A method formanufacturing a printed wiring board comprising the steps of: forming athrough-hole, or a non-through-hole, using an energy beam in a boardmaterial of plate form, or sheet form, composed mainly of either athermoplastic resin or a thermosetting resin containing uncuredcomponent, or a mixture of both, or in a board material of plate form,or sheet form, made of woven, or nonwoven, fabric impregnated with amaterial composed mainly of either said thermoplastic resin orthermosetting resin containing uncured component, or a mixture of both;and forming connection means in said through-hole, or non-through-hole,provided as a result of said hole-forming step, for electricallyinterconnecting one surface and the reverse surface of a board material;wherein said board material in storage on shelf before said hole-formingstep is carried to the hole-forming step within a storage time period inwhich the water content in said board material does not exceed 1% interms of weight percentage.
 21. The method for manufacturing a printedwiring board recited in claim 20, wherein said board material before thehole-forming step is laminated at least on both surfaces with sheet filmof high anti-moisture permeability until it is carried to thehole-forming step.
 22. The method for manufacturing a printed wiringboard recited in claim 21, wherein said board material before thehole-forming step is laminated on both surfaces with sheet film of highanti-moisture permeability that has a size larger than the boardmaterial and the sheet films are sealed together by heat in a placeouter than said board material, in order to have the board materialisolated from the outside atmospheric air until it is carried to thehole-forming step.
 23. The method for manufacturing a printed wiringboard recited in claim 21, wherein the sheet film of high anti-moisturepermeability is polyethylene terephthalate.
 24. The method formanufacturing a printed wiring board recited in claim 21, wherein thesheet film is provided with a metal film deposited on the surface. 25.The method for manufacturing a printed wiring board recited in claim 24,wherein the metal film is of aluminum.
 26. The method for manufacturinga printed wiring board recited in claim 1, wherein the energy beam is alaser beam.
 27. The method for manufacturing a printed wiring boardrecited in claim 26, wherein the laser beam is a CO₂ laser beam.
 28. Themethod for manufacturing a printed wiring board recited in claim 1,wherein the process of forming connection means includes filling of thethrough-hole, or non-through-hole, provided as a result of thehole-forming step with a paste containing conductive particles.
 29. Themethod for manufacturing a printed wiring board recited in claim 1,wherein the process of forming connection means includes a process inwhich the board material filled with a paste containing conductiveparticles is provided with a metal foil on one or the both surfaces andthe board material provided on one or the both surfaces with the metalfoil is compressed together by thermal compression.
 30. The method formanufacturing a printed wiring board recited in claim 1, wherein theprocess of forming connection means includes a process in which thethrough-hole, or non-through-hole, provided as a result of thehole-forming step is plated with a conductive metal.
 31. The method formanufacturing a printed wiring board recited in claim 1, wherein thethermosetting resin is an epoxy group resin.
 32. The method formanufacturing a printed wiring board recited in claim 1, wherein thewoven, or nonwoven, fabric is made mainly of an organic fiber material.33. The method for manufacturing a printed wiring board recited in claim32, wherein the organic fiber material is mainly of an aromaticpolyamide fiber.
 34. An equipment for manufacturing a printed wiringboard comprising a unit of forming a through-hole, or anon-through-hole, in a board material for a printed wiring board usingan energy beam generated from an energy beam generator, whereindehumidifying means is provided in a section for feeding said boardmaterial.
 35. The equipment for manufacturing a printed wiring boardrecited in claim 34, wherein the dehumidifying means is a hot air dryingunit.
 36. The equipment for manufacturing a printed wiring board recitedin claim 35, wherein a temperature of the hot air drying unit is set notto be lower than 50° C., and within a limitation of temperature and timeperiod in which the gel time of board material does not shift.
 37. Theequipment for manufacturing a printed wiring board recited in claim 34,wherein the dehumidifying means is a vacuum drying unit.
 38. Theequipment for manufacturing a printed wiring board recited in claim 37,wherein the vacuum drying unit does not accompany heating.
 39. Theequipment for manufacturing a printed wiring board recited in claim 37,wherein the vacuum drying unit accompanies heating.
 40. The equipmentfor manufacturing a printed wiring board recited in claim 39, wherein aheating temperature of the vacuum drying unit is set to be lower thanboiling point of solvent used in the resin of board material.
 41. Theequipment for manufacturing a printed wiring board recited in claim 37,wherein vacuum level of the vacuum drying unit is set to be not lowerthan 100 Torr.
 42. The equipment for manufacturing a printed wiringboard recited in claim 34, wherein the dehumidifying means is capable ofdehumidifying at once two or more pieces of board materials stacked updirect together.
 43. The equipment for manufacturing a printed wiringboard recited in claim 34, wherein the dehumidifying means keeps watercontents in a board material to be not more than 1% in terms of weightpercentage.
 44. An equipment for manufacturing a printed wiring boardcomprising a unit of forming a through-hole, or a non-through-hole, in aboard material for printed wiring board using an energy beam generatedfrom an energy beam generator, wherein means for suppressing moistureabsorption into said board material is provided either in a section forfeeding said board material or a hole-forming section, or in both of thesections.
 45. The equipment for manufacturing a printed wiring boardrecited in claim 44,. comprising a unit of forming a through-hole, or anon-through-hole, in a board material for printed wiring board using anenergy beam generated from an energy beam generator, wherein means forsuppressing moisture absorption into said board material is providedeither in a section for feeding said board material or a hole-formingsection, or in both of the sections.
 46. The equipment for manufacturinga printed wiring board recited in claim 44, wherein the means forsuppressing moisture absorption is a unit which stores a board materialon shelf in low humidity atmosphere.
 47. The equipment for manufacturinga printed wiring board recited in claim 46, wherein the low humidityatmosphere is a vacuum state.
 48. The equipment for manufacturing aprinted wiring board recited in claim 47, wherein the vacuum state is ahermetically sealed space which is evacuated after a board material islodged in.
 49. The equipment for manufacturing a printed wiring boardrecited in claim-44, wherein the low humidity atmosphere is a sealed, orcirculating, atmosphere substituted with dry nitrogen.
 50. The methodfor manufacturing a printed wiring board recited in claim 44, whereinthe process of suppressing moisture absorption keeps contents of waterin a board material to be not more than 1% in terms of weightpercentage.
 51. The method for manufacturing a printed wiring boardrecited in claim 44, wherein the low humidity atmosphere is anatmosphere where the partial pressure of water vapor is not larger than10 mmHg.
 52. A material for a printed wiring board of plate form, orsheet form, composed mainly of either a thermoplastic resin or athermosetting resin containing uncured component, or a mixture of both,or a material for printed wiring board of plate form, or sheet form,made of woven, or nonwoven, fabric impregnated with a material composedmainly of either said thermoplastic resin or thermosetting resincontaining uncured component, or a mixture of both, wherein watercontents in said material for printed wiring board is not more than 1%in terms of weight percentage.
 53. The material for a printed wiringboard recited in claim 52, wherein said material is suitable to thehole-formation with an energy beam.
 54. The material for a printedwiring board recited in claim 52, wherein the thermosetting resin is anepoxy group resin.
 55. The material for a printed wiring board recitedin claim 52, wherein the woven, or nonwoven, fabric is mainly of anorganic fiber material.
 56. The material for a printed wiring boardrecited in claim 55, wherein the organic fiber material is mainly of anaromatic polyamide fiber.